Which of the Following Describes a Mandatory Requirement for a Group

Which of the following describes a mandatory requirement for a group of undertakings that wants to appoint a single data protection officer?

A) The group of undertakings must obtain approval from a supervisory authority.
B) The group of undertakings must be comprised of organizations of similar sizes and functions.
C) The data protection officer must be located in the country where the data controller has its main establishment.
D) The data protection officer must be easily accessible from each establishment where the undertakings are located.